underground

1(alsothe Underground)(British English)(North American Englishsubway)[singular]an underground railway/railroad system in a cityunderground stationsthe London UndergroundI always travel by underground.British/​Americanunderground / subway / metro / tube A city’s underground railway/​railroad system is usually called the underground (often the Underground) in British English and the subway in North American English. Speakers of British English also use subway for systems in American cities and metro for systems in other European countries. The Metro is the name for the systems in Paris and Washington, D.C. London’s system is often called the Tube.comparemetro,tubeSee related entries:Trains,Train and bus travel

2the underground[singular + singular or plural verb]a secret political organization, usually working against the government of a countrySee related entries:Terrorism

3(Indian English)a person who works against the government as a member of a secret political organization

Extra examplesI always seem to get lost in the underground.I often travel on the underground.It brought the whole underground system to a halt.We went by underground.